Exhaust manifold questions

[clhyer]

Good morning,
I have two sets of exh. manifolds to choose from, an older set of A body 318's and a later model set from an '88 M body 318. The first question is what are all the extra holes on the later manifold. They were all plugged up from crap and could not have helped anything. The extra holes are circled in the first pic.

In the second pic i have circled the end of the heat riser. If I remove the heat riser can I leave this bushing in and simply braze it shut or will I have to remove the bushing and have someone else weld the cast iron hole up? Would I be better off just to wire the heat riser open?

Third, On the M body there was an O2 sensor on the top of the manifold. So far I've not found a plug that will fit. Anyone know what size thread this is and where I might find a plug? It is not a 3/8" pipe plug.

Last, for now anyhow, anyone use a set of these M body manifolds and have any fitment issues? They are a good bit larger than the older manifolds and should have less restriction.
Thanks,
C

 

[4spdragtop]

Hey C, with the 02 sensor hole, cant you put the 02 sensor in a hole guage and get the size of it?? Take the 02 sensor to the car store with ya and see what they come up with. Sorry cant help with the other questions. Dont even know if I'm much help with the 02 question either.

 

[RedFish]

I think most of the hole questions are answered with straight bolt threads although they may be metric. I would concentrate on the fitment before speeding time or money on them. Reason being if the M body manifolds fit in the a-body we would all know it by now. Seems Dakoda/Durango manifolds are the most popular alternative for fit and flow.

 

[clhyer]

Well, I have the answer to 1 of the 4 questions. The O2 sensor should be a metric 18mm x 1.5 thread pitch. Summit carries this plug:
http://www.summitracing.com/parts/SUM-G2990-1/
Our local ACE carries an oil drain plug with these same measurements, I will pick one up there, for about $3.00.

Still hoping for the answers to the other 3 questions...
C

 

[mtandrews]

i don't know, unless you can get someone to weld (braze???) the holes closed for next to nothing I would be looking for some pre emission manifolds. I found a pair here on FABO for $25 which kept me from Jury Rigging up something....